Modem service station fuel dispensers monitor the amount of fuel delivered to a customer's vehicle by counting the number of pulses in a digital pulse stream generated by a device commonly called a "pulser." This arrangement is illustrated in the schematic representation of a dispenser 40 as shown in FIG. 1. The pulser 10 typically is mounted on the dispenser meter 8 and is connected to the meter 8 via a mechanical linkage. This arrangement is conventional in the field. The pulser 10 generates an electronic signal such as a digital pulse stream which has a pulse rate that is proportional to the flow rate of the product flowing through the dispenser during a particular fueling operation. Each pulse represents a known volume, so that the pulse rate is proportional to the volumetric rate. This information is processed by the dispenser electronics 14 to create a customer display of the total volume of product dispensed and possibly to control the certain dispenser operations such as blending processes.
In some parts of the world incidences of tampering with this pulse stream have occurred. This tampering consists of installing an additional signal generator 11 or "black box" between the pulser 10 and the dispenser electronics 14 to add false pulses to the pulse stream before it is received in the dispenser electronics 14. The effect of this pulse stream modification is to cause the customer to be overcharged for his purchase. Because the pulse stream is a very simple signal, creating the electronics necessary for this type of deception is straightforward.
There is a need for a modification to the pulser 10 and the dispenser electronics 14 to detect when the deception described above has taken place, to alert operating personnel and, if necessary, halt fueling operations.